MR-9: Add membership expiry warning banner for members

[zhickson]

Summary

This PR introduces a new membership expiry banner that helps members avoid interruptions by warning them before access lapses.

When enabled, logged-in members will see a clear banner if their membership has expired, expires today, or is due to expire soon. The warning window is configurable in plugin settings, so site owners can choose how early to remind members.

What's included

• New optional Expiry Banner feature in Memberful settings.
• Configurable reminder window (1-90 days, default 7).
• Front-end banner messaging for:
  • expired memberships
  • memberships expiring today
  • memberships expiring in N days
• Dismiss control so members can hide the banner for the current session.
• Accessibility support (role/aria-live) and extension hooks for custom implementations.

Scope and behaviour

• Shown only to logged-in members when criteria are met.
• Not shown to admins.
• No impact when feature is disabled.

Test plan

• Turn on the expiry banner in settings.
• Set threshold days (e.g. 7) and save.
• Verify banner appears for eligible members with correct message states:
  • expired
  • expires today
  • expires in N days
• Verify no banner for members outside threshold.
• Verify no banner for admins.
• Verify dismiss hides the banner for the current session.
• Turn feature off and verify banner no longer appears.